1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, for controlling the engine based on the most fuel-efficient torque.
2. Description of the Related Art
A conventional control system for an internal combustion engine of this type is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2006-193137. This control system controls an internal combustion engine installed on a hybrid vehicle, based on operating lines which represent torque minimizing a fuel consumption ratio with respect to the rotational speed of the engine (most fuel-efficient torque), while performing control for learning and updating the operating lines. Specifically, the control system sets in advance an operating line for each set of environmental conditions, such as an outside air temperature, atmospheric pressure and humidity, on a coordinate plane in which the torque and rotational speed of the engine are set as coordinate axes, and stores each operating line as one of a plurality of predetermined operating lines. Further, during operation of the engine, the control system calculates the fuel consumption ratio based on detected torque of the engine and the rotational speed and fuel injection amount thereof. When the calculated fuel consumption ratio is smaller than a fuel consumption ratio indicated by an operating line associated with the set of environmental conditions at the time, the operating line is updated using the calculated fuel consumption ratio.
As described above, in the above conventional control system, the engine is controlled based the predetermined operating lines. Therefore, particularly when the operating lines are set on an environmental condition set basis, it is required to set in advance a large number of operating lines, which increases the number of setting steps to thereby increase costs of the control system. Further, unless the set of environmental conditions are met, an operating line associated with the set of environmental conditions is not learned, and hence the engine is sometimes controlled based on an unlearned operating line using the most fuel-efficient torque deviated from an optimum value. In this case, fuel economy is degraded.